Process of forming passivating internal in2o3 bands in silver-indium alloys



pr 23, 1968 R. A. RAPP ET A1.

PROCESS OF FORMING PASSIVATING INTERNAL INaOJ BANDS IN SILVER-INDIUM ALLOYS Filed. Oct. SO, 1963 GMM.

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United States Patent O 3,379,579 PROCESS F FORMING PASSIVATING INTERNAL 111203 BANDS IN SILVER-INDIUM ALLOYS Robert A. Rapp, Westerville, and Darrell F. Frank, Springfield, Ohio, assignors to the United States of merica as represented by the Secretary of the Air orce Filed Oct. 30, 1963, Ser. No. 320,235 2 Claims. (Cl. 14S-6.3)

ABSTRACT 0F THE DISCLOSURE Process forming continuous layer of corrosion impassible barrier of In2O3 below and parallel to the surface of refractory metals and alloys and the article produced by making a silver-indium alloy sample in the range of from 8.74 to 15 mol percent indium, welding thermocouple hot junctions to the opposite surfaces of the sample and holding the sample temperature at 500 C. in air for from 2 to 6 hours in forming the indium-oxide corrosion-passivating band below the surface of the sample used to arrest both mechanical and chemical deterioration of the matrix below the band.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to any of us of any royalty thereon. r

This invention concerns the arresting of metal deterioration by developing a metal oxide passivating band within the metal matrix of a body, in both article and in method.

Metals are weakened and are destroyed by their continued reactivity with materials and atmospheres in their environment. Alloys of silver, copper, nickel, iron, vanadium, chromium, hafnium, refractory metals, and the like, that contain low percentages of less noble alloying elements such as aluminum, beryllium, indium, etc., are subject to internal oxidation when exposed tooxygen, or other oxidizing atmospheres, and particularly at elevated temperatures.

The application of non-corroding metal, organic or oxide coatings on the surface of an oxidizable material serves to arrest oxidation only as long as the coating is not impaired by wear, scratches, abrasion, etc., that expose the underlying metal to its environment.

In comparison with surface coating practice, the disposition Well within the body of an oxygen impervious layer that is protected by the overlying body from abrasion, wear, scratches and the like, and such that its function is unimpaired by changes in surface dimensions, prolongs the strength and the service life of the body.

The present invention is the development of the disclosed method and article that embodys a corrosion impassable barrier below the surface of and within the matrix of the metal.

An object of this invention is to provide a metal corrosion barrier well within the matrix of the metal as a positive barrier to the deterioration of all of the metal beyond the barrier.

A further object is to materially prolong the strength, the period of service, the life and the integrity of devices made of metals that are used in environments that destructively combine with the metal.

Silver offers a high solubility and diffusivity for oxygen atoms without the formation of silver oxide. Illustratively, if a dilute silver-indium alloy is held at a high temperature in air, the indium in the alloy reacts with dissolved oxygen atoms to form indium oxide within the silver ice matrix which contains the reactants and the products of the reaction.

i. 2 In-i-S O In2O3 At the steady state, this reaction occurs in very narrow reaction front, which is parallel to the outer surface of the piece and as it is formed proceeds inwardly with increasing time.

In the accompanying drawings:

FIG. 1 is a graph illustrating steady-state concentration profiles for an internally oxidized silver-indium alloy;

FIG. 2 is a schematic graph of concentration profiles for oxygen and indium at various times during a temperature-time program designed to stop the movement of the reaction front; and

FIG. 3 is a graph of the calculated temperature-time programs for the internal passivation of illustrative silverindium alloys.

An illustrative slab is a silver 8.74 mole percent indium alloy. Because of the high dilusivity of oxygen in silver, the steady-state concentration gradient of oxygen in the alloy is very nearly linear. Experimentally, in a temperature-time program for a silver, range of from 8.74 to l5 mole percent indium alloy held initially at 773 K. or 500 C. for two hours in air, a compact, oxygen irnpervious layer of In2O3 of the thickness delta forms at a depth Xi below the surface of the alloy.

The formation of corrosion-passivating internal indium oxide bands in silver-indium alloys is an illustrative example for silver base alloys containing a small amount of a less noble alloying element. The internal oxidation reaction front is held motionless while the precipitation of the oxide continues by operation of the temperaturetime program that is disclosed herein. An internal oxide band so made of the thickness delta, serves as a passivating barrier to further internal oxidation.

The mathematical derivation of the temperature-time program is calculated to stop at a prescribed depth in the material movement of the internal oxidation reaction front in silver indium alloys of Nm 0.l5. A pure In203 band is formed while the reaction front is stopped. These internal bands of In203 serve to passivate from further internal oxidation the silver-indium alloys of Nm 0.l5 beyond the bands.

In FIG. l of the accompanying drawings is shown a schematic representation of the steady-state concentration profiles for oxygen and the alloying element in indium in the silver-indium alloy experiencing isothermal internal oxidation. N11, and No are the mole fractions of indium and of oxygen respectively, along the ordinate, at any time t that exceeds the reaction starting time to, plotted against the depth x from the outer surface, along the abscissa. The value Xi is the constant depth from the surface of the alloy at which the reaction front is lixed by stopping its movement. At steady state, the flux of oxygen arriving at x=Xi equals 1.5 times the flux of indium.

At the steady state, the flux of oxygen arriving at the depth x=Xi of the piece is one and one-half times the ux of indium. The diffusion coefficients DO for oxygen in silver and DIn for indium in silver, are exponential functions of temperature, with the activation energy for the diffusion of indium equalling nearly four times that for oxygen. Due to this difference in the temperature dependence of diffusion, if after an initial period of heating at a constant temperature, the temperature is raised according to suitable temperature-time program, the reaction front is established at a constant depth Xi. The resulting compact layer of 111203 at the prescribed depth Xi passivates the alloy from further internal oxidation. The suitable temperature-time program is determined mathematically.

The mathematical derivation of the temperature-time program that is calculated to stop the movement of an internal oxidation reaction front in silver-indium alloys of Nm less than 0.15 mole, appears in Review, Society for Industrial and Applied Mathematics, P O. Box 7541, Philadelphia, Pa., volume 5, No. 1, pages 67 to 72 Jauuary 1963. The summary of the experimental results is presented in a preprint entitled, The Formation of Passivating Internal In2C3 Bands in Silver-indium Alloys by Robert A. Rapp, Darrell F. Frank, and Iohn V. Armitage. This report was submitted for publication in Acta Metallurgica on Aug. 9, 'i963 and subsequently published May 1964 in vol. 12, pp. 505-513. Experimentally successful data were derived from the accompanying graphs with a reasonable degree of approximation.

The silver-indium experimental first sample alloy of the composition NIW- 0.0874 has the calculated passivation band of pure 111203 of the thickness delta=5-6 105 cm., positioned at the Xi depth of 2.05 3 crn. below the surface of the alloy. This sample is formed or developed after a 7,200 second or 2 hour time with its preprogram oxidation at 500 C. This sample was 100% effective in its passivation control of the oxidation of the alloy below the passivation band.

The silver-indium alloy experimental second sample of he same N1n=0-0874 composition has the calculated passivation band of pure In2O3 of the thickness delta :10.0X10-5 cm., positioned at the Xi depth of below the surface of the alloy. This second sample is formed or developed during a 21,600l second or 6 hour time t with its preprogram oxidation at 500 C. This second sample was also 100% effective in its passivation control of the oxidation of the alloy below the passivation band.

In FIG. 2 of the accompanying drawings, the concentration protiles for oxygen and indium are drawn schematically for several times to, t1, t2, etc., during a temperaturetime program fixing the internal oxidation reaction front distance from the outer surface of the Ag-In alloy Xi equal to the Xi subzero.

In FIG. 2 the steady-state mole fraction of oxygen, indicated as NO is plotted along the ordinate and the depth or distance from the outer surface of the alloy and indicated by the symbol x. Xi represents the depth or distance of the oxygen impervious internal band from the outer surface of the sample. In the body of the graph N0 indicates the mole fraction of oxygen; NIU indicates the mole fraction of indium; Nm@ indicates the mole fraction of l InzOg in the bulk alloy; and No@ indicates the mole fraction of oxygen at the outer surface of the alloy. The lower case t indicates time; DO indicates the diffusive ity of oxygen in silver; and Dm indicates the diffusivity of indium in silver.

In FIG. 2 the concentration profiles of oxygen and indium are drawn schematically during a T-t program fixing Xi=Xi0, whereas before upper case T represents a temperature dependence and lower case t represents time.

In the above-cited Rapp, Frank and Armitage publication, the temperature-time program is designed to stop the movement of the internal In2O3 oxidation arresting front at Xi() after a preprograrn oxidation in air at T0=773 K. which corresponds to 500 C.

In FIG. 3 of the drawings are disclosed calculated temperature-time programs for the alloy Nrn-:0.0874 and to is 7,200 seconds or 2 hours, and 21,600 seconds or 6 hours preoxidation periods. Integrating the ux of indium arriving at the reaction front determines the thickness of the pure In2O3 band that precipitates and blocks the further penetration of oxygen into the silver-indium alloy. Thickness of the In2O3 band for the 0.0874 alloy is calculated by integrating between 500 C. and 880 C.

The diiiusivity of indium in silver is a function of the indium content. indium reaches the reaction front by Cil A diusing down a steep gradient between NIn-:Nmm and NlnzO. indium diffuses substitutionally in silver.

The experimental samples are made of alloys that are melted under a borax flux in a resistance furnace, rolled into ribbons and then are recrystallized and homogenized at 700 C. The specimens or samples are free from rolling cracks near the surface and have minimized grain boundaries, both of which can temporarily hold up an advancing front, and can cause the internal oxidation barrier to deviate from being parallel to the outer surface.

Preparatory to oxidation, the specimen surfaces are electro-etched with l normal nitric acid. The ends of two small chromel-alurnel thermocouples are spot-welded to the specimen so that about Ms separates the welds of each pair of wires of a couple, thereby making the specimen between the welds the hot junction of the thermocouple. The two therniocouples are spot-welded on the opposite faces of the specimens at the opposite ends of the rectangular ribbon specimens, that illustratively may be 1 cm. in length.

The two pairs of thermocouples support the specimen or sample midway between banks of radiant heat retlectors. One thermocouple is attached to a temperature control'ing device and the other thermocouple is attached to a millivolt recorder.

The temperature controlling device is programmed by an electromechanical curve-following device with a variable time base. A servo-driven pickup probe of the curve-following device is continuously positioned by the plotted temperature-time function attached to a rotating drum. The comparison of the temperature of the specimen thermocouple with the program temperature causes the temperature controling device to regulate the voltage supplied to the radiant heat reflectors. In this manner, the temperature of the specimen is held constant according to the prescribed temperature-time program.

An illustrative temperature controlling device to which one thermocouple is attached is the Ignitron temperature controller Model 4046, programmed by a Data Track function generator Model PGE, `both by Research Incorporated. An illustrative millivolt recorder is the x-y millivolt recorder by Honeywell Electronik.

The specimen or sample, that is positioned between the two banks of heat reflectors is preoxidized in air at 500 C. for 2 to 6 hours for accomplishing the initial internal oxidation of silver-indium alloys. The choice of the lower preoxidation temperature insures a steep initial indium gradient and wide temperature range for the passivation program. The process here of interest comprises the growing in air of an oxide corrosionpassivating band below and substantially parallel to the surface of a silver-indium alloy sample containing in the range of about from 8.74 to 15 mole percent indium specimen, electroetching the sample with one normal nitric acid and polishing its surface, welding an end of each leg of a rst thermocouple directly to the sample so that about one-eighth inch separates the welds of each wire of a couple, welding an end of each leg of a second thermocoupe directly to the sample on an opposite face and on an opposite end of the sample; supporting by means of the thermocouples the sample between a bank of radiant heat reectors; attaching the cold end of the wires from the first thermocouple to a temperature controller programmed by a function generator electromechanical curve-following device provided with a servo-driven pickup probe and with a variable time base; attaching the cold end of the wires from the second thermocouple to an x-y millivolt recorder; causing the pickup probe of the function generator to be continuously positioned Iby the plotted temperature-time function attached to the surface of a rotating drum; comparing the temperature of the sample thermocouple to the temperature demanded by the program and regulating the temperature of the sample to be held constant at a controlled val-ue of about 500 C. for a time period of from 2 to 6 hours in forming an indium oxide corrosion-passivating band at a predetermined depth below the surface of the silver-indium alloy sample.

Following the preoxidation period, the sample is healed in conformity with the calculated temperaturetime program and is held at temperature for one hour, thereby forming the In203 Corrosion-passivating -band within the sample. Oxidations of the specimen are accomplished at 500 C. to 850 C., etc., as indicated by the curves in FIG. 3 of the drawings for two temperature-tirne programs.

It is to be understood that the embodiments of the present invention that are disclosed herein in structure, in mode of operation and in results are successful operative embodiments of the present invention, and that comparable substitutions and modifications in materials, in process steps and in apparatus that are disclosed herein, may be made without departing from the spirit and the scope of the present invention.

We claim:

1. The process of growing in air a metal corrosionpassivating band belowand substantially parallel to the surface of a silver-indium alloy containing about 8.74 mole percent indium comprising the steps of electroetching the sample with nitric acid and polishing its surface; preoxidizing the alloy in air at 500 C. for 6 hours to eieet an initial internal oxidation of the alloy; and then raising the temperature of the alloy from 500 C. to about 880 C. for a period of 350 seconds.

2. The process of growing in air a metal corrosionpassivating band below :and substantially parallel to the surface of a silver-indium alloy containing about 8.74 mole percent indium comprising the steps of electroetching the sample with nitric acid and p-olishing its surface; preoxidizing the alloy in air at 500 C. for two hours to effect an initial internal oxidation of the aloy; and then raising the temperature of the alloy from 500 C. to about 880 C. for a period of 100 seconds.

References Cited Rapp, R. A.: The Transition from Internal to EX- ternal Oxidation and the Formation of Interruption Bands in Silver-Iridium Alloys, in Acta Metallurgica, 9(8), pp. 730-741 August 1961.

ALFRED L. LEAVITT, Primary Examiner.

I. R. BATTEN, JR., Assistant Examiner. 

